[libc++] Improve the tests for vector::erase (#116265)

In particular, test everything with both a normal and a min_allocator,
add tests for a few corner cases and add tests with types that are
tri

[libc++] Improve the tests for vector::erase (#116265)

In particular, test everything with both a normal and a min_allocator,
add tests for a few corner cases and add tests with types that are
trivially relocatable. Also add tests that count the number of
assignments performed by vector::erase, since that is mandated by the
Standard.

This patch is a preparation for optimizing vector::erase.

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